This invention relates to fuel pellets produced from biomass material, in particular agricultural fibre.
There is a growing interest in biomass materials, such as wood, wood byproducts, annual plants, etc. as a source of energy. For instance, wood residues such as sawdust are quite widely available in the form of small pellets. Wood residues from sawmills and wood processing industries typically have less than 1% mineral content. Wood comprises primarily three structural components: Cellulose (45-50% by weight), hemicelluloses (20-25%), and lignin (20-30%). Cellulose is a long, straight chain homopolymer (d.p. 5,000-10,000) consisting of anhydro d-glucopyranose linked via [beta] 1,4 glycosidic bonds. Hemicelluloses have a lower degree of polymerization (150-200) and may be relatively straight or branched. These consist variously of five- and six-carbon sugars. Although the type and amount of hemicellulose in wood varies with species, most hardwoods have a predominance of glucuronoxylan, consisting of a linear backbone of xylopyranose with a 4-0-methylglucuronic acid residue on approximately 10% of the xylan rings. Softwoods primarily contain galactoglucomannan consisting of [beta]-D-mannopyranose, [beta]-D-glucopyranose, and [alpha]-D-gal actopyranose. Cellulose and hemicelluloses contain free hydroxyl groups that lend wood its inherent hygroscopicity. Lignin is a large, amorphous polymer consisting of varying ratios of the phenyl propane precursors linked mainly (>⅔) by ether bonds and the rest by C—C bonds.
Other materials such as alfalfa, switch grass, etc. have been studied as energy sources in the form of pellets. There are a number of advantages to biomass fuel pellets including, low levels of dust, free flowing material, high energy density and uniform burning rate. In order to meet the requirements of a first quality pellet fuel, the pellets must contain less than about 1% by weight ash and have a heat output of at least 8,000 BTU per pound of fuel.
The exterior protective surface of whole grains, seeds, fruits and nuts typically have what is called a hull or shell, sheath or husk and may be referred to as the bran of the whole grain and seed e.g., corn bran, oat bran, rice bran, soy bran.
When comparing the energy content at the Bodycote testing labs at Pointe Clair Quebec, the hulls from corn kernels, oats, soybean, canola, wheat, barley for example, were found to be comparable to wood biomass, typically in the range 7,000 to about 9,000 btu's per pound (dry basis). In fact the oil seeds far exceeded this level reaching over 12,000 BTU's per pound of fuel.
The moisture content of most of the hulls derived from whole grains and seed is typically between 7 and 26% mc, depending on the type of processing employed in the harvesting and processing. Typical field dried seeds and whole grains could be lower in moisture content. If they are harvested at below 16% moisture content, they may not require additional drying, otherwise they are susceptible to the formation of mold during storage.
The wet milling of corn for the production of ethanol for example, producescorn fiber. This is usually dried in a rotary drum dryer and mixed with what is called a syrup (the protein content derived from the fermentation process) and is sold as an animal feed called DDG's (dried distillers grains). This process is very similar to the production of value added food grade products, also derived from the wet milling of corn. These products include corn sweetners and corn starch used in a multitude of food preparation applications. This process also produces fibre and protein (steep water) which are mixed together and sold as animal feed.
The major difference between the agriculture and wood based materials were in the mineral content and type as a percentage. In the case of corn bran, the ash or mineral content can be below 1% by weight or as high as 7% or more for oat hulls and as high as 20% or more for rice hulls.
Verrecchia et al. U.S. Pat. No. 5,375,540 describes a combustion system capable of burning fuel pellets and also discusses the problems associated with trying to burn many natural biomass materials, including whole corn. The inventors acknowledge that these fuels present a serious problem of clinker formation from ash. They attempted to solve the problem by modifying the design of the burner.
Clinkers refer to the formation of lava-like pieces formed in the fire pot of pellet stoves. These are formed when minerals and salts contained in the pellets are exposed to the high burning temperatures of the pellets. The process is known as ash fusion. Alkali in the ash of annual crop biomass fuels can create a serious fouling problem and a particularly troublesome alkali is potassium.
Another difficulty with biomass fuel pellets is the formation of pellets that will resist crumbling. Some wood components are capable of self bonding during the pelletizing process, while other woods require the addition of a binding agent. Many other natural cellulosic materials, such as agricultural wastes also require an additional binding agent. Other differences between wood biomass and agriculture fibre is the hemicellulose as a percentage, with agriculture hulls usually containing higher levels. The hulls of agriculture materials usually contain varying levels of hemicellulose 12% to about 40% and corn bran has one of the highest levels typically 30%-40% or more. Hemicellulose is similar to cellulose but is less complex and is the second most abundant polysaccharide in nature. Hemicellulose consists mainly of sugars and sugar acids and can be found in wood or corn fibers.
Lignin is the binder that flows during the production of wood pellets, the hemicellulose remains bound in the cell wall. Because there is a lower level of Lignin in the hulls of agriculture fibers, binder additives must be used to produce good quality, stable pellets. This is undesirable because the binding agents raise the cost and they may include components that when burned form atmospheric pollutants and or promote clinker formation. In Johnston et al. U.S. Pat. No. 4,529,407, as well as in Jesse U.S. Pat. No. 5,342,418, pellets are made from a mixture of natural cellulosic material, such as bark mixtures, and a synthetic polymeric thermoplastic material as the pellet forming feedstock. There are many examples of the use of such thermoplastic materials to bind together the cellulosic particles which make up the final pellet.
Like starch, fiber is made up of carbohydrate polymers with sugars as their basic building blocks, but the structure of fiber is more varied and complex. Cellulose is an insoluble fiber that resembles amylose starch with its linear structure of D-glucose units, but is formed by beta-instead of alpha-1,4 linkages. Hemicellulose is a general term for the noncellulose fiber fraction of plant cell walls. The basic structure of hemicellulose is a main chain with side chains attached, both of which can be made from a variety of five and six carbon sugars. Pentosans are the part of the hemicellulose fraction where the main chain is made up of five carbon sugars, usually D-xylose. Pentosan fractions can be soluble or insoluble, depending among other factors on their degree of polymerization. The hemicellulose fraction is usually described as insoluble.
Fiber is what gives plant foods structure and varying textures. Fiber comes in two forms, each with its own important qualities. Soluble fiber, so named because it can dissolve in water, is made of pectins, gums, and mucilages. Insoluble fiber does not dissolve in water and consists of cellulose, hemicellulose, and lignins. Most foods contain both types of fiber, although some foods are more predominant in one form.
The hulls of typical whole grains and seeds like soybeans, represent about 8 to 10% of the weight of soybean grain (Sessa and Wolf, 2001; R. Glahn, personal communication). In the case of the Soybean, Mullin and Xu (2001) reported the following major constituents of the hulls, on a dry weight basis:
cellulose14 to 25 g/100 ghemicellulose14 to 20pectin10 to 12protein9 to 12uronic acid7 to 11ash4 to 5lignin3 to 4
Thus soybean hulls, though high in fiber, are a significant source of protein, comparable to corn grain in the amount of crude protein.
The primary use for soybean hulls is feed. Hulls are routinely removed during crushing of soybeans but are returned to the processing stream to be added to the meal fraction. The meal fraction which contains most of the protein, adds as much as 4%-5% or more of ash to the final end product. In the case of corn bran the steep water added over 6% to the ash content of the final material (Bodycote 2001 test report). Excess hulls may be sold as feedstuffs or discarded as waste. Removal of hulls costs processors 5 to 10 cents per bushel.
In the corn wet milling process, the yellow skin of the corn kernel is dissolves off during the process creating fiber and a steep (protein) liquid. The steep and corn bran as two separate streams are recombined at about a 70% fiber to 30% steep protein. This also increases the ash content, so that these pellets can not be used in typical pellet stove combustion processes. The higher the chlorides and potassium salts, the higher the occurrence of clinkering during the combustion process. Potassium and Chlorides as well as other minerals are apparently tied up as organically bound elements, or in forms that are readily volatilized during combustion. A portion of the potassium occurs as dissolved salts in inherent moisture, cations attached to carboxylic and other functional groups, complex ions, and chemisorbed material. A substantial fraction (15%-40%) of this potassium volatilizes during combustion (Baxter 1994). They act as the glue that holds silica to silica, forming a mass of slag or clinker, which leads to a host of other negative combustion issues.
It is an object of the present invention to develop fuel pellets from natural biomass materials which will be high quality pellets produced without the need for additional bonding agents such as polymeric thermoplastic materials or special combustion apparatus to handle ash fusion issues